1. Technical Field
This invention relates to a method and apparatus for attenuating acoustic waves (often termed “ring around” acoustics) propagating through the walls of a pipe for a clamp-on ultrasonic flow meter.
2. Background Information
Clamp-on ultrasonic flow meters are a desirable tool for determining characteristics of a fluid flow traveling through a pipe. The flow meters, such as that shown in FIG. 1A, typically include a plurality of ultrasonic sensors, each having a transmitter (TX) and a receiver (RX). In some sensor configurations, ultrasonic signals emitted from a transmitter travel through the immediate pipe wall, the fluid flow disposed within the pipe, and through the opposite pipe wall where they are sensed by a receiver. In other sensor configurations, transmitters and receivers are disposed on the same side of the pipe; the sensed fluid flow signal component is one that has reflected off of the opposite pipe wall and traversed the fluid flow a second time. Regardless of the sensor configuration, the received signal is processed to determine information such as flow velocity, volumetric flow rate, water cut, etc.
One of the primary challenges associated with clamp-on ultrasonic flow metering is distinguishing the fluid borne signal component from a structural borne component that is generated when the ultrasonic signal travels through the pipe on a path substantially normal to the surface of the pipe. FIG. 1A diagrammatically illustrates a clamp-on flow metering arrangement having a transmitter (TX) and a receiver (RX) operable to transmit and receive signals that include a fluid borne signal component 20 and a structural borne component 22. The fluid borne component 20 contains useful information relating to characteristics of the fluid flow 24. The structural signal component 22, on the other hand, does not travel through the fluid flow 24, and therefore does not contain useful information relative to the fluid flow 24. In fact, the structural borne signal 22 is predominantly a shear wave that travels within the pipe wall 26, and can “ring-around” the pipe circumference several times before dissipating. Collectively, the structural borne “ring-around” signals 22 create interference that makes it difficult to extract the desired fluid borne signal component 20. Hence, there is considerable value in minimizing or eliminating structural borne signal components 22.